1. Field of the Invention
The present invention relates to a non-volatile IC memory (hereinbelow simply called as non-volatile memory) and more specifically relates to an improvement of a large capacity rewritable non-volatile memory represented by such a flash memory which requires data erasing by a block unit to a non-volatile memory which does not necessitate the data rewritting by a block unit.
2. Description of Related Art
A non-volatile memory includes a non-rewritable mask ROM and a rewritable PROM, and the PROM includes a rewritable after erasing EPROM and a rewritable after electrically erasing EEPROM.
The advantages of the EEPROM are that it endures rewritings for a relatively great number of times (100.about.100,000 times) and enables rewriting on-board. However most of the EEPROMs are a plurality of transistors per cell type of which storage capacity per a predetermined area is poor in comparison with that of a single transistor per cell type such as the mask ROM and an erasable with ultraviolet EPROM.
It is a current demand to increase memory capacity for resolving the above mentioned drawback of the EEPROMs, which single transistor per cell type EEPROMs have been studied to reduce them to practice and as one of the typical examples thereof a so called inclusively erasable flash memory is now reduced to practice.
The problem with regard to the capacity is resolved by changing the memory structure from that for a plurality of transistors per cell type to that for a single transistor per cell type. However, as reflections thereof advantages associated with the constitution of the plurality of transistors per cell type memory are lost. For example, the single transistor per cell type memory includes no selection transistor such that it is generally impossible to specify an individual cell and to erase only the data in the specified individual cell. In other words, when data erasing is required for the purpose of such as rewriting, it is necessary to erase an entire memory cell in the IC or, in case of a memory type wherein the inside of the memory is divided into a plurality of blocks, it is necessary to erase memory cells by a block unit.
In these kinds of the single transistor per cell type non-volatile memories, the data rewriting can not be performed by a bit unit or by a word unit (usually 4 or 8 bits) but has to be performed by at least a block unit (usually more than several thousand bits). This requirement presents a new restricting condition to the users of these non-volatile memories. Thereby the selectability thereof by the users is narrowed, and such is one of the reasons to prevent a spreading use of the flash memory.